Organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs.
The organic layers in these devices are usually composed of a polycyclic aromatic hydrocarbon. Substituted naphthacenes is one class of fluorescent materials useful in the manufacture of EL devices. The naphthacene known as rubrene, or 5,6,11,12-tetraphenylnaphthacene, is commercially available and can be prepared by reacting 1,1,3-triphenylpropargyl alcohol with thionyl chloride and heating the resulting product in the presence of an organic hindered amine base. However, the yields of rubrene prepared in this manner are usually low, not reproducible and contain impurities. Rubrene, prepared in this manner, must be subjected to extensive purification techniques to render it sufficiently pure to be useful in EL devices. In addition, when substituents are present in the starting propargyl alcohol, formation of naphthacene isomers occurs.
Because rubrene and its derivatives are very prone to photo-oxidation, the normal purification techniques of re-crystallization and chromatography are not easily applied to the purification of the crude material from the reaction. Precautions have to be taken to eliminate the presence of oxygen or light. Impurities and isomers from the preparation procedure, and also the photo-oxidation products or endoperoxides as they are known, that contaminate rubrene or other naphthacene derivatives give rise to EL devices with unacceptable performance. Even very small amounts of impurities, such as 1% or less, can cause significant problems in EL devices.
Moureu et al., C.R. Acad. Sci. (1926), Vol. 182, 1440; Moureu et al., Bull. de la Soc. Chim. de Fr. (1930), Vol. 47, 216; Wittig et al., J. Fur Praktische Chemie, (1942), Vol. 160, 242; Rigaudy et al., Tetrahedron (1977), Vol. 33, 767; and Essenfeld, U.S. Pat. No. 4,855,520 refer to the preparation of rubrene in yields ranging from 20-50% and employ different techniques to purify the material.
Moureu et al., in C.R. Acad. Sci. (1926), Vol. 182, p. 1441, describes the preparation of rubrene from 3-chloro-1,3,3-triphenylpropyne by heating this material from 71° C. to 120° C. in the absence of solvent. The purification and removal of impurities from the crude material requires an involved procedure of treating with different solvents.
Moureu et al., Bull. de la Soc. Chim. de Fr. (1930), Vol. 47, p. 217-220 does not describes the preparation of rubrene but describes the influence of factors such as dilution and catalysts on the formation from 3-chloro-1,3,3-triphenylpropyne. The conclusion is that the best procedure for the preparation of rubrene from 3-chloro-1,3,3-triphenylpropyne is by heating the material in the absence of solvent. The purification and removal of impurities from the crude material requires a very involved procedure of treating with different solvents including the high boiling solvent, naphthalene.
Wittig et al., J. Fur Praktische Chemie, (1942), Vol. 160, p. 244 also describes the preparation of rubrene from 3-chloro-1,3,3-triphenylpropyne by heating this material under vacuum to 120° C. in the absence of solvent. Again, the purification and removal of impurities from the crude material requires an involved procedure of treating with different solvents.
Rigaudy et al., Tetrahedron (1977), Vol. 33, p. 773, describes the preparation of rubrene from a cyclobutane derivative.
Essenfeld, in U.S. Pat. No. 4,855,520 describes a long and involved procedure for the preparation of naphthacenes in the presence of a hindered amine base, and reports a yield of 37%. The procedure calls for the use of several different solvents. Careful removal of the initial low boiling solvent from the reaction mixture is followed by the careful addition of a second solvent with a high boiling point. Hindered amine bases are disadvantageous in manufacturing processes because they are oftentimes expensive and not environmentally safe, requiring special handling and disposal procedures. Furthermore, when substituents are introduced into the procedure of that described by Essenfeld, naphthacene isomers are formed. Isomer formation is undesirable in materials to be employed in EL devices because each isomer will have different chemical and physical properties and would have to be separated into single components before they could be used in the EL devices. Separation into single isomers or components requires additional effort and is often difficult.
The stability and luminance performance of these fluorescent materials in EL devices in general, tends to improve when fabricated from single materials with high purity. There is a continuing need in the EL industry for new, short, environmentally friendly and simple procedures for the preparation of single isomer, high purity naphthacenes. Devices fabricated from naphthacenes with low purity give poorer performing EL devices and limit the applications of these EL devices.
The problem to be solved therefore is to provide a simple procedure that would yield high purity single naphthacene compounds. Such procedures should require minimum exposure to light and oxygen and which could be applied to the preparation of naphthacenes with a variety of substituents.